Vulcanicity in East Africa

Vulcanicity or vulcanism is the process by which gases and molten rock from the interior of the earth are intruded into the earth's crust or extruded onto its surface.

When the molten material, that is magma, is intruded into the crust, it solidifies, forming intrusive features. While 'when it is extruded onto the surface of the earth, it solidifies to form extrusive features. These are the features referred to as volcanic landforms.

Volcanic mountains

Volcanic mountains are formed through the process known as Volcanicity. Volcanicity is the process by which materials in all forms such as liquids, gases and solids are forced into or onto the earth’s surface forming Intrusive and Extrusive land forms respectively. The hot liquid, which is brought from the earth’s mantle, is known as Magma before it reaches the surface and it is referred to as Lava when it reaches the surface.

Volcanoes can be active, dormant, or extinct.

Magma and other materials are kept below the earth’s surface because of great pressure and they have very high temperatures going beyond 1000°C. When the temperature reduces, the materials come to the surface to form volcanic features and pressure normally reduces when folding and faulting take place. The magma finds it’s way out through the Vent.

Mount Gahinga and Mount Muhabura in the East African Rift valley.

Vocabulary

When the magma reaches the surface, features formed are called extrusive volcanic features and they include the following; Volcanoes, Volcanic Plugs, Lava Plateau, Caldera, Crater, Hot Springs, Ash and Cinder cones.

Structure of a volcano.

A cross section of a composite volcano reveals alternating layers of rock and ash: (1) magma chamber, (2) bedrock, (3) pipe, (4) ash layers, (5) lava layers, (6) lava flow, (7) vent, (8) lava, (9) ash cloud. Frequently there is a large crater at the top from the last eruption.

The other associated features to a volcanic mountain are:- Geysers, calderas, Lava Dammed Lakes, hot springs, lava plateaus, ash and Cinder Cones, Cumulo Dome, Volcanic Plugs, Explosion Craters among others.

Ash and cinder cones

When magma solidifies within the earth’s crust, the features formed are referred to as intrusive volcanic features. They include the following: Sills, Dykes, Batholiths, Lacolith, Lapoliths, and phacoliths.

The lava which comes to the surface is of three main types namely, Acidic lava which is very sticky does not flow fast and it solidifies quietly, basic lava, which is fairly fluid and flows gently; intermediate lava, which has characteristics of both acidic and basic lava.

(a) A’a lava spread over large areas. (b) Pahoehoe lava tubes (c) P¯ahoehoe lava is less viscous than a’a lava so its surface looks is smooth and ropy. (d) Pillow lava.

A volcanic mountain or a volcano or volcanic cone is a hill or mountain formed by the eruption of molten rock from a central opening in the crust. The materials build around the vent in form of a cone. As long as magma continues to come, a volcano grows and expands. Types of volcanoes include the composite volcano, made of layers of lava and ash, each layer depicting an eruption.

Secondly there are the Ash and Cinder cones, which are concave shaped cones with very high peaks.

Extrusive (volcanic) features

1. Explosion craters: These are small or medium-sized depressions on the surface of the earth formed by explosive eruptions. Explosion craters are usually round-shaped, small-sized and flat-floored.

Some craters are dry while others contain water, and hence are crater lakes. Several craters exist in Kasese, Kabarole and Bushenyi districts in Uganda.

The Bunyaruguru field is said to have well over 120 of them. A fine example is Lake Katwe situated close to Lake Edward. Others are Nyamunuka, Nyamusingire and Kyamwiga.

In Kenya a number of craters occur just south of Lake Turkana. These are the Likaiyu craters averaging 24 metres in width. In Tanzania, small explosion craters occur within the vicinity of Oldonyo l'Engai near Lake Natron.

2. Ash and cinder cones:

These are steep-sided hills formed by violent eruption and composed of small-sized volcanic rock particles. Several cones exist in Kisoro district in Uganda and include Muganza, Shozi, Sagitwe and Bisalo. In Kenya, the Likaiyu cinder cones are found south of Lake Turkana.

volcanic mountains

· Mt Kilimanjaro ( Tanzania)

· Mt Kenya ( Kenya)

· Mt Elgon ( Uganda + Kenya )

· Mt Muhavura - Uganda

· Mt Meru ( Tanzania)

· Mt Moroto, Napak etc ( Uganda)

3. Composite cones:

A composite cone is a very large volcano built out of alternating layers of lava and ash. These materials reach the surface through a single vent. A composite cone is built up over a long period of time as a result of repeated explosive and quiet outpourings of material from the earth's interior. There are several examples of composite cones. These include Mount Muhabura on the Uganda-Rwanda border, Mount Kenya and Mount Kilimanjaro.

Calderas:

A caldera is a wide depression or a 'big crater'. One way in which a caldera may form is through the blowing off of the top of a volcano by a violent volcanic eruption. When this occurs, a wide and big depression is created at the top of the remaining cone. Examples in East Africa include Napak in Karamoja and Ngorongoro in northern Tanzania.

Lava plateaus:

A lava plateau is an upland with more or less monotonous relief and consisting of successive layers of lava. Examples are the Yatta plateau in eastern Kenya and the Kisoro lava plateau. Extensive lava flows covered part of Kisoro, forming an undulating surface, sometimes with some ash and cinder cones rising above the general surface.

Geysers and Hotsprings:

Geysers are natural fountains that throw up jets of hot water and steam at regular intervals through a vent in the surface. In some areas, rainwater seeps through cracks in the rocks and drains into a crevice or a large cave-like chamber so deep that it reaches hot rocks. The heat of these rocks comes from the molten rocks below. Eventually, the intense heat boils the water, which then turns into steam. This increases the pressure inside the crevice as bubbles of steam build up.

Finally, the pressure is strong enough to shoot the water and steam upwards and out through a vent, high into the air. When the jet has died down, the crevice fills with new water and the process repeats.

Hot springs are gushes of hot water that are found on the land surface. As molten materials deep in the earth cool down, they give off water vapour and carbon dioxide. This hot vapour then finds its way upwards through the cracks in the rocks, cooling as it goes, until it condenses to become water. Finally, it gushes from the ground as a hot spring. This water may be pure and clear, but it is rich in mineral salts dissolved from the rocks it has passed through on its way to the surface. As water cools around the edges of the spring, the minerals form crystals and grains. These build up into beautiful rock layers shaped like waves, basins and terraces. Hot springs are found at the southern end of Lake Bogoria in Kenya. The Sempaya hot springs are found in Uganda's western district of Bundibugyo. In Tanzania, hot springs include Maji-Moto.

Let us Watch the video below. Take note of the Magma, lava and the resultant features formed internally and externally

Intrusive features

Intrusive features are igneous materials that solidify below the surface of the earth. They remain there until weathering and erosion expose them. It is only then that they outcrop on the surface to form distinct landforms. The various intrusive features include batholiths, dykes and sills.

Batholith

A batholith is a very large intrusion in the earth's crust. It covers a large area and often has steep sides. It descends down to great depths within the earth's crust. When the rocks covering the batholith are removed through weathering and erosion, it is exposed and appears on the surface of the earth in the form of a hill or upland. Batholiths are found in Mubende district in Uganda, the Maragoli area of western Kenya and in northern Tanzania.

Dyke

A dyke is a vertical intrusion within the earth's crust that is formed when magma rises vertically across rock strata. The magma later solidifies to form hard material. Dykes exist in eastern Uganda and in the Sukulu hills of Tororo, south and west of Lake Turkana and southeast of Kisumu in Kenya.

Sill

Sills are horizontal igneous rocks that lie between rock strata. The magma rises from the interior of the earth and spreads horizontally along the rock strata.

Laccolith

Laccolith is a dome-shaped intrusion but with more or less flat base. It is formed when magma rises and solidifies within the crust before reaching the surface. It is formed from viscous magma which is unable to spread far. The magma accumulates in a large mass forcing the overlying rocks to bend upwards.

Lopolith

Lopolith is another intrusive feature resulting from volcanicity. This is a feature with a 'saucer' or 'bowl' shape. The magma rises through the crust and then spreads horizontally between rock strata. As the magma spreads along the bedding planes, its weight depresses the underlying rocks. Consequently, the overlying rocks bend downwards resulting in a bowl-shaped surface. The shape of the lopolith is therefore largely attributed to the weight of the molten material that depresses molten rocks beneath.

Map of East Africa showing Intrusive and Extrusive landforms

How volcanic mountains are formed

Formed by extrusive volcanic activity due to convective currents of the interior.

Extrusive volcanic activity involves the movement and ejection of molten rocks or magma from the central vent in the crust and deposited onto the earth’s surface.

It’s made up of alternate layers of ash and lava

These materials are erupted and build upwards around the vent to form a cone shaped mountain

The alternate layer of ash and lava materials were due to repeated eruptions beginning with ash layers

The ash eruption were explosive due to gas content in it

When the main vent is block with magma, minor pipes ( dykes) are formed leading to development of parasitic cones on the mountain side

During violent eruption, the top of the volcano may be blown off leading to the formation of a crater at the top. See diagram below

Diagram showing formation of a volcanic mountain

c) The effect of volcanic mountains on the following ;

(i ) rainfall

· The windward sides of high mountains have more rainfall

· The leeward sides of a mountain have less rainfall

· Lower slopes tend to have less rainfall

(ii) temperature

· Areas of high altitude tend to have low temperatures because temperatures reduce with increase in altitude low land areas have high temperatures ( lower slopes of volcanic mountains)

(ii) humidity

· The leeward side has less or low humidity

· The windward side of mountain slopes have high humidity

· Lower slopes of mountains have low humidity

(d) Advantages of volcanic mountains to people living around mountains

· provide beautiful scenery that attract tourists

· provide avenues for recreational activities like mountaineering, hiking etc

· The existence of forests has promoted forestry

· The different vegetation types on these mountains provide good habitat for different animals

· They induce the formation Orographic rainfall

· They are source of rivers

· The water from the rivers is used for irrigation

· The water from these rivers is also used for domestic and industrial purposes

· Mountain evolution is associated with the formation of valuable minerals

· Gentle slops are used for crop farming or settlement

· There is wild life conservation

· Promote research or education purposes

· Areas of little rainfall ( rain shadow) provide pasture for grazing

· Good volcanic fertile soil for agriculture

· Production of geothermal power on the geysers and hotsprings as it the case of Ol karia, south west of Lake Naivasha.

Disadvantages

· volcanic mountains are barriers to the development of transport and communication lines

· Steep slopes promote soil erosion and land slopes

· Mountains act as climate barriers ( leeward side of mountain)

· Volcanic eruptions can be destructive to life and property

· Mountains reduce cultivable land

· The forests on these mountains act as good sites for rebels and other bad elements

· Infertile thin layers of soil on steep slopes due to erosion.

Influence of Vulcanicity on human activities

The positive side of vulcanicity

Soil formation: When the lava rocks are weathered, very fertile soils .are produced. The Kenya highlands, for example, have rich volcanic soils which are suitable for the growth of various crops. Similarly, Mount Elgon is well known for being a region where the fertile volcanic soils have promoted growth of arabica coffee and bananas. The slopes of Mount Kilimanjaro are also famous for cultivation of crops such as coffee and tea.

Mining: Volcanic activity may result in the formation of valuable minerals. Some of these minerals occur in igneous rocks, for example salt in Lake Katwe in Uganda, and diamond at Mwadui, in northern Tanzania.

Quarrying: Plutonic igneous rocks are resources often excavated and crushed to yield aggregate that is used for building and other constructional purposes.

Energy: Volcanic activity is closely related to geothermal energy. When water percolates through the ground, it sometimes comes into contact with hot magma which may happen to be near the surface. The water is consequently heated and rises to the surface in the form of hot springs and/or geysers. The force of the steam jets can then be harnessed to generate geothermal power. This is already being done in Kenya at 01 Karia, a place near Naivasha. In Uganda, there are numerous hot springs in the western arm of the rift valley which constitute a high potential for geothermal power generation.

Tourism: number of volcanic mountains provide magnificent scenery which attracts tourists. Some of the highest mountains in Africa, namely Mount Kilimanjaro (5,895 metres) and Mount Kenya (5,199 metres) are of volcanic origin. These snow-capped mountains within the vicinity of the equator attract many tourists throughout the year. On the slopes of Mount Kenya, for example, is the famous Mount Kenya National Park.

Rainfall: Volcanic eruptions have led to the formation of high mountains in some parts which directly influence the formation of relief rainfall. As moisture-laden winds blow across these mountains, they are compelled to rise and in the process cool down. The moisture contained condenses, giving rise to rainfall on the windward side. The rainfall is a source of domestic, industrial and agricultural water.

Source of rivers: The heavy rainfall and ice on some mountains have influenced the evolution of the radial drainage pattern. Mountains are therefore important sources of rivers which can be used for irrigation, domestic and industrial purposes. Examples of rivers originating from volcanic mountains include Sironko, Manafwa and Turkwell on Mount Elgon.

Forestry: The high mountains which receive high rainfall are associated with forest vegetation on their slopes. The trees are being conserved through forestry or selectively utilised on a sustainable basis.

Benefits of the mountains to the people of East Africa

· beautiful scenery attracts tourists

· provides avenues for recreational activities like mountaineering

· The different vegetation on these mountains provide good habitat for different animals

· There is forestry in the area

· They induce the formation Orographic rainfall

· They are sources of rivers e.g. River Nyamwamba, Sebwe, Mobuku, Lume and so on from Rwenzori mountains

· The water from the rivers is used for irrigation e.g. Mobuku irrigation scheme.

· The water from the rivers is also used for domestic and industrial purposes

· Mountains evolution associated with the formation of valuable minerals e.g. tin and wolfram in Kigezi

· Gentle slopes are used for crop farming or settlement

· The wild life conservation e.g. Mt Rwenzori National park

· Promote research or education purposes

· Areas of little rainfall ( rain shadow) provides pasture for grazing

· Good fertile soils for agriculture e.g. Mt Elgon area

The negative side of vulcanicity

Obstacle to road development: It is difficult and expensive to construct roads across highland areas. Some parts of the highlands in Kigezi and Bugisu in Uganda either have few and poor roads or are totally inaccessible.

Soil erosion and landslides: Steep slopes of volcanic mountains have higher chances of experiencing soil erosion and landslides than gentle lowlands. This is common in Kigezi and the Kenya highlands. These dangers occasionally damage agricultural land and/ or destroy property and life. Manjiya county in Mbale district, Uganda, has often experienced landslides that have caused a wide range of social and economic problems.

Obstacle to settlement: The tops of some volcanic mountains limit settlement because they are so high above sea level that they are too cold and sometimes glaciated. The temperatures are too low to favour human settlement. The tops of the Kilimanjaro and Kenya mountains are clear examples.

Obstacle to mechanised agriculture: The steep slopes of mountains are an obstacle to mechanised agriculture. It is very difficult for tractors and other farm equipment to work the soil where the gradient is steep, such as on mountain sides.

Problems faced by the people living in the mountainous areas

· Mountains are barriers to the development of transport and communication lines

· Steep mountain slopes promote soil erosion leading to infertile soils.

· They promote landslides leading to loss of human life and valuable property.

· Mountains act as climatic barriers ( leeward side of the mountain)

· Volcanic eruptions can be destructive to life and property

· Mountains reduce cultivable land

· The forests on these mountains act as good sites for rebels and other bad elements

· High cost of constructing roads and houses.

· Sketch map of East Africa showing block mountains, volcanic mountains and escarpments

 Study Activity 1

1.  Study the two diagrams below showing  block and volcanic mountains and answer questions that follow;

a)  Name the features marked;

P, Q, R ,S and T on a block mountain and features  a, b, c, d, e and f on a Volcanic mountain

b) Describe how a block mountain is formed

c) Draw a sketch map of East Africa showing at least the major block and volcanic mountains

d) Outline the economic benefits of block mountains to the surrounding areas

 Study Activity 2

2.  a)      Draw a sketch map of East Africa and on it mark and name

(i) the East African rift valley

(ii) mountains Rwenzori and Usambara

(iii)        Lakes: Naivasha and Eyasi.

(b)          Describe the processes which was responsible for the formation of the East African Rift Valley.

c)  Explain the benefits of the Rift Valley to the people of East Africa.

d) Outline the problems faced by the people living in the Rift valley of East Africa

 Activity 3

3. a) Name any:

(i) two volcanic mountains

(ii) one block mountain in Each East African country

(b)     with the aid of sketch diagrams, explain how a volcanic mountain is formed.

(c)     How has the volcanic mountains influenced the following in the areas where they are located?

(i) rainfall  (ii) temperature  (iii) humidity

(d)     State the advantages and two disadvantages of a volcanic mountain to the people living in and around it.

 Activity 4

4. (a) Draw a sketch map of East Africa and on it mark and name only two

i) Block mountains

ii) Volcanic mountains

iii) Escarpments

(b)     Describe how block mountains mentioned in a) I) above were formed.

(c)     Giving specific examples, outline the economic benefits of mountains to the surrounding areas.

(d)     State ant three problems that are faced by the people in the areas where mountains are found.